On the basis of organic light emitting diodes (OLEDs), novel area light elements can be realized. As two-dimensional illuminants whose luminance is moderate as compared to that of LEDs, OLEDs are ideally suited for manufacturing two-dimensional diffuse light sources. Said light sources are predicted to see a tumultuous development similar to that of displays based on OLEDs. Due to their thin-layer technology, OLEDs may also enable, in the distant future, realizing flexible illuminants that allow entirely new applications in terms of illumination of spaces, or rooms.
Since OLEDs are current-operated devices, an important issue in the manufacture of large-area light emission elements is homogeneous current density distribution on large areas. Said homogeneity is limited by the necessity of at least one transparent contact, which is normally realized by transparent conductive oxides (TCOs) or transparent metal layers. Due to the low conductivity of said TCOs, said layers limit the maximum light emitting surface area.
Solar cells based on organic materials (OPVCs) are comparable, in terms of architecture, to OLEDs; however, due to the organic materials used, said structures enable optical radiation to be converted to electrical current. Said current may be drained off via the contacts. In this case, too, the transparent electrical contact reduces the maximally usable device size.
A known architecture of an OLED or OPVC comprises, in addition to a substrate having a transparent electrode base layer of ITO, ZnO, for example, substrate metallization, insulating layers, the stack of the organic functional layers (HTL, insulation, emitter, ETL) as well as a metallic roof electrode. A cover glass that is provided with cavities on the inside is adhered to the substrate by means of an encapsulation adhesive and seals the functional layers toward the outside.
Both with organic light emitting diode areas (OLEDs) and with organic solar cells (OPVCs), a high filling level (ratio between active surface area and overall surface area) is a very important criterion for usability. With OLEDs and/or OPVCs, said filling level is limited by two factors.
The first factor is encapsulation. Encapsulation is typically effected at least by means of a thin-layer encapsulation protecting the layer construction from air and air humidity. What is most widely employed and useful for utilization as a light source in marketable products is a mechanical protection of the organic layers that can be realized only via encapsulation by means of a cover glass. Said cover glass is adhered to the substrate in the non-active edge region of the OLEDs or OPVCs, typically by means of a UV-curable adhesive. Said adhesive region, which cannot be minimized to any extent desired in order to ensure a barrier effect toward air and to ensure mechanical stability, is not available for the active light emitting area.
The second factor is the useful electric contacting of the OLED and/or OPVC, which is also effected in the edge region, but outside the cover glass. Contact pads are typically deposited within said region, metal conductor lines leading from said contact pads to the electrodes underneath the cover glass.
Those two factors, adhesive area and contact pads, lead to a reduction in the active area of the OLED and/or OPVC and to a significant non-luminous edge with OLEDs, and to a non-light-absorbing edge with OPVCs.
In addition to a comparatively low filling level, this results in that, when several OLED elements are lined up in order to achieve a large light emitting area, there will be a clearly visible non-luminous grid.
A standard design of an OLED and of a solar cell, or OPZV, comprises a transparent ITO layer (indium tin oxide) as a top electrode on glass having a thickness of about 100 nm, an organic layer (sometimes comprising up to 7 sub-sheets) having a thickness of about 100-200 nm, and a metallic cathode (in most cases made of aluminum) having a thickness of about 100-500 nm; the respective layer thicknesses are limited and cannot be increased to any amount desired so as to achieve, for example, a lower sheet resistance of the electrode layers. One variant of this design in case of utilizing non-transparent substrates is to use a transparent top electrode (thin metal or ITO) to achieve light being coupled out and/or in via the top electrode.
With large-area devices, the high resistance of the transparent layer (about 10-100 ohm/square), i.e. of the ITO layer, or top electrode, leads to inhomogeneity of the power input, since the contacts of the layer are possible only on the edge of the light emission element. Thus, the maximum size is limited to about 50×50 mm2.
To achieve larger light emitting areas, in particular metal reinforcements in the form of webs are introduced into the transparent layer. Said metal meshes (also referred to as busbars or grids) reduce the effective sheet resistance in accordance with their packing density, and thus enable realization of larger light emitting areas.
Due to their non-transparency, said grids, or webs, reduce the effective surface area of the device. For this reason, metal grids, or metal meshes, of up to about 25% of the ITO surface area are actually useful. A useful improvement would be to increase the grid metal thickness, or the thickness of the lines of the metal mesh, which is not useful, however, due to the structuring possibilities and layer thicknesses of the organic layers.
The external contacts of the OLED/OPVC elements are connected to a distribution board via spring contacts or similar electric contacts. Since it is via these contacts that the total current for the anode and the cathode is supplied or drained off, the contact may be divided into at least two. In order to achieve homogeneous light distribution with this configuration, a lateral wide contacting line is useful, which reduces the active light emitting area, or the optically active area.
One alternative to edge contacting is rear-side contacting, wherein in the event of an OLED emitting through the substrate, an insulating layer is applied over the roof electrode, and a further metal layer is applied on said insulating layer, said metal layer being connected to the transparent electrode underlying the organic layers via a through-connection.
However, since the cover glass is still useful, even in this case, an inactive region for adhering the cover glass to the substrate is useful. In addition, a contacting area is also useful on at least one side of the OLED, which increases the non-luminous edge region on said side.
A further problem are the cover glasses for encapsulating the OLED/OPVC elements, which may have cavities in order to receive the functional layers and absorption materials. Said cavities can be produced only by means of production processes (generally by etching) requiring a large amount of resources, which is reflected in a high unit price, in particular with large-area OLED/OPVC elements. In addition, the production process has a high potential of damaging the environment due to the etching chemicals used.